1. Technical Field
The present invention relates to a novel copolymer that is useful as a binder resin or a pigment dispersion resin of an aqueous ink, and an aqueous ink and an ink cartridge that include the copolymer.
2. Background Art
The inkjet recording system is advantageous in that, compared with other recording systems, the process is simple, full-color printing is easily achieved, and high-resolution images can be obtained even with an apparatus having a simple configuration. For this reason, the inkjet recording system is becoming common and spreading from personal use to office use and fields of commercial printing and industrial printing. In such an inkjet recording system, aqueous ink compositions containing water-soluble dyes as colorants are mainly used. However, the water-soluble dyes have drawbacks of having insufficient water resistance and light resistance. Accordingly, pigment inks that contain water-insoluble pigments instead of water-soluble dyes are being studied.
In inkjet printing for office use, conventional paper sheets are mainly used as recording media and high image density is required. In general, when printing is performed to apply pigment ink onto a conventional paper sheet, the pigment does not stay on the surface of the paper sheet and permeates into the paper sheet; and, as a result, the pigment density on the surface of the paper sheet becomes low and the image density becomes low. By increasing the pigment concentration of the ink, the image density can be increased. However, the viscosity of the ink is increased and discharge stability is degraded.
Immediately after pigment ink is discharged onto a conventional paper sheet, water in the ink causes swelling of the surface of the paper sheet and the difference in extension ratios of the front and back surfaces of the paper sheet becomes large, resulting in a problem of curling of the paper sheet. This phenomenon disappears as the paper sheet dries. Accordingly, the phenomenon has not been regarded as a problem in low-speed printing. However, with the increase in the printing speed, recording media that remain in a curled state due to printing need to be transported. As a result, a problem of a paper jam may be caused. In order to prevent this, a method of adding a penetrating agent to the ink may be effective to allow quick permeation of water into paper sheets; however, the ink becomes hydrophobic and hence it becomes difficult to ensure the storage stability of the ink; in addition, the permeability of the pigment into the recording media is also increased and the image density is further decreased.
In order to address such problems, various techniques for making pigment stay on the surfaces of paper sheets have been proposed. For example, JP-2009-513802-A proposes an inkjet ink that contains a liquid vehicle, a colorant, and a polymer having at least one functional group having a specific calcium index. This proposal states that a monomer forming the polymer is 4-methacrylamido-1-hydroxybutane-1,1-diphosphonic acid; when the colorant is brought into contact with a paper sheet, the colorant is destabilized by the diphosphonic acid group of the polymer and Ca salts in the paper sheet to thereby improve images. However, the ink has a problem of having low storage stability.
JP-2012-051367-A proposes an inkjet recording method of making a receiving solution containing a Ca salt adhere to paper sheets and performing printing with an ink containing a pigment to which a phosphorus-containing group is bonded, a resin emulsion, and a surfactant. This proposal states that the Ca salt of the receiving solution reacts with the phosphorus-containing group to thereby achieve improvements in terms of feathering and fixability. However, in recording on conventional paper sheets, the effect of increasing the image density is not sufficiently provided.
In the fields of commercial printing and industrial printing, there is a demand for techniques that allow stable formation of images having higher resolution and definition at a higher speed. Recording media used are, for example, conventional paper sheets, coated paper sheets, art paper sheets, and impermeable films such as PET films. Accordingly, there is a demand for ink having high versatility for such recording media.
When images having high resolution and definition are formed at a high speed on various recording media, in particular, slightly permeable media such as coated paper sheets and art paper sheets or impermeable films, beading (unevenness) tends to be caused. To address this, JP-H3-160068-A proposes an ink that has a minimum film forming temperature of 40° C. or more and at least contains, as fixing agents, wax emulsion, resin emulsion, latex, organic ultrafine particles, and inorganic ultrafine particles.
JP-2006-188624-A proposes an inkjet ink at least containing water, an organic solvent, and a polymer, wherein the polymer contained in the ink or in the ink from which a portion of water has been evaporated exhibits UCST-type phase separation in the range of 0° C. to 100° C.
JP-2008-536963-A proposes an ink containing a jettable vehicle; a plurality of pigment solids self-dispersed in the vehicle; and a half-ester of styrene maleic anhydride (SMA) dispersed in the vehicle.
In addition, JP-2012-52027-A proposes a method of changing rheological behavior of ink in response to pH change by using a pH-responsive polymer including plural hydrophilic segments in side chains and plural pH-responsive segments that become hydrophilic or hydrophobic in response to pH values.
However, when the existing inks described above are used, it is difficult to form images on various recording media at a high speed without causing beading (unevenness).
As in the inkjet recording system described above, aqueous pigment inks containing pigments as colorants in writing implements such as marking pens, ballpoint pens, and plotter pens have a problem of exhibiting low color density on conventional paper sheets, in particular, on white conventional paper sheets, compared with dye inks. In order to address this problem, JP-2005-298802-A proposes an aqueous pigment ink containing O/W emulsion of water-insoluble (meth)acrylic resin and/or styrene-(meth)acrylic acid having an average particle size of 50 to 200 nm and a minimum film forming temperature of 50° C. or more, urea or a urea derivative, water, pigment, an aqueous resin, and a phosphate surfactant. However, the effect of increasing the color density is not sufficiently provided.